Treatment of water and waste water by electro-coagulation is well known as a process by which contaminant particles can be coagulated to form precipitates which can be subsequently separated using conventional flocculation, settlement or filtration systems.
Conventional coagulation is a chemical process in which the charged particles in colloidal suspension are neutralized by mutual collisions with counter ions. After the particle is neutralized, it will attract to other colloidal particles and agglomerate to form precipitates. It is generally accepted that coagulation occurs because of a reduction of the net surface charge of the particle to a point where electrostatic repulsive forces are reduced, then allowing van der Walls forces to dominate and allow particle agglomeration. Agglomerated particles could be separated by a conventional separation technique such as settling clarification tanks. Chemicals have been used to coagulate contaminants in both water and waste water treatment systems. These chemicals are not only costly; they also contribute secondary pollution to the environment.
Electro-coagulation is an electrical process in which a pair of electrodes are used to neutralize small charged particles in colloidal suspension. The electrodes (anode and cathode) are subjected to a specific current density. Upon oxidation, the anodes are oxidized and form metal ions (either Fe+2, Fe+ or Al+3) in solution that react with hydroxide (OH−) anions created in the electro-coagulation process. This leads to the formation of metal hydroxide ions, either cationic or anionic species depending on the pH of the waste water. A combination of inert anodes and metal (titanium) cathodes can also be used. The inert electrodes accomplish pollutant destabilization utilizing the transfer of electrons within the electrolyte. The transfer of electrons and formation of protons (H+) created in the electro-coagulation process can effectively destabilize a range of metal and organic pollutant species.
Under appropriate conditions, various forms of charged hydroxyl (OH−) and Al+3 species might be formed. These gelatinous hydroxo cationic/anionic complexes can effectively destabilize pollutants by adsorption and charge neutralization by enmeshment of the particle, thus forcing it to react with a counter ion. Pollutants are also destabilized by ions of opposite charge (e− and p+) produced during electro-coagulation. Particles that undergoing destabilization, will agglomerate due to the attractive van der Wall forces and form into a stable precipitate which could then be separated by conventional separation technique. Typical chemical reactions at both the aluminium anode and cathode are shown below:
Anode:Al(s)→Al3+(aq)+3e−(lose electrons)Al3+(aq)+3H2O→Al(OH)3+3H+nAl(OH)3→Aln(OH)3n Cathode:2H2O+2e−→H2(g)+2OH−Al3++3e−→Al(s) (gain electrons)
The electrochemical dissolution of the aluminum anode produces Al3+ ions which further react with OH− ions (from cathode), transforming Al3+ ion initially into Al(OH)3 and then into the gelatinous hydroxyl precipitate (Aln(OH)3n). Depending on the pH of the aqueous medium, different ionic species will also be formed in the medium such as: Al(OH)2+, Al2(OH)22+, and Al(OH)4. At the cathode, hydrogen (H2) gas and hydroxide (OH−) ions are formed from the division of H2O and dissolved metals are reduced to their elemental state.(i.e. Al+3).
The electrochemical dissolution of the iron anode produces iron hydroxide, Fe(OH)n where n=2 or 3. There are two proposed mechanisms for the production of the iron hydroxide. Like the gelatinous aluminum hydroxyl precipitate (Aln(OH)3n), the iron hydroxide precipitate (Fe(OH)n) formed remains in the aqueous medium (stream) as a gelatinous suspension. This suspension can also remove water and waste water contaminants either by complexation or by electrostatic attraction, followed by coagulation. The cathode is subject to scale formation, which can impair the operation of the system. Typical chemical reactions at both the iron anode and cathode are shown below:
Anode:4Fe(s)→Fe2+(aq)+8e−(lose electrons)4Fe2+(aq)+10H2O(I)+O2(g)→4Fe(OH)3(s)+8H+(aq) Cathode:8H+(aq)+8e−→4H2(g)Overall:4Fe(s)+10H2O(I)+O2(g)→4Fe(OH)3(s)+4H2(g) Anode:Fe(s)→Fe2+(aq)+2e−(lose electrons)Fe2+(aq)+2OH−(aq)→FeOH2(s) Cathode:2H2O(I)+2e−→H2(g)+2OH−(aq) Overall:Fe(s)+2H2O(I)→Fe(OH)2(s)+H2(g) Electro-Coagulation Treatment (ECT) Systems
Utilizing electro-coagulation treatment (ECT) systems to treat waste water was practised through most of the 20th century with limited success. Within the last decade, technological advances in ECT systems has proved that it is an effective treatment method, brought on partially by increased environmental regulations and environmental awareness. ECT is used to remove a variety of water and waste water contaminants such as heavy metal ions (chromium, zinc, silver), suspended solids and small colloids (greases and oils). Typically, the ECT system must have optimized operational parameters (pH, current density, and temperature)
Examples of such electro-coagulation systems are shown in U.S. Pat. No. 6,139,710 (Powell) issued Oct. 31, 2000, U.S. Pat. Nos. 6,294,061 and 5,928,493 (Morkovsky) assigned to Kaspar Electroplating and issued Sep. 25, 2001 and Jul. 27, 1999, and in U.S. Pat. Nos. 5,439,567 and 5,108,563 (Cook) assigned to Environmental Systems and issued Aug. 8, 1995 and Apr. 28, 1992 respectively.
Many such systems use parallel plates as the necessary electrodes. Cook discloses an arrangement in which a central rod forms an anode and the cathode is defined by a surrounding sleeve which is perforated with a series of holes so as to allow the water to enter the area inside the cylindrical cathode so as to be acted upon between the anode and the cathode.
One significant problem which arises with continuous flow ECT systems is treating a sufficient rate of flow of water and waste water at reasonable cost, while at the same time preventing the system from clogging due to the formation of precipitates and corrosion scale deposited on the cathode.
It is essential therefore for continuous flow that the arrangement of the cathode and anode, herein referred to as the electrolytic cell, be in effect self cleaning in that the flow of liquid is sufficient to carry with it the coagulated precipitates while allowing treatment of the liquid at a sufficient efficiency to remove the contaminants.
Up till now there has been no suitable design of an electrolytic cell which provides an adequate treatment of the water and waste water to continuously remove the contaminants while at the same time preventing fowling of the cell. Once fowling commences, this builds up until the cell becomes clogged. There is at this time no effective way to self-clean the cell once the coagulation of particles has commenced. While electro-coagulation is therefore known and accepted in principle, its commercial continuous flow application has been limited by this problem.